In the most widely employed form of photography, images are captured by a photographic element comprised of a support and at least one emulsion layer comprised of radiation-sensitive silver halide grains. The radiation-sensitive grains are prepared by reacting halide ions with silver ions in a dispersing medium. Silver chloride, silver bromide, and silver iodide are known to be useful alone or in combination to form the radiation-sensitive grains.
Silver iodide grains exhibit .beta. or .gamma. phase crystal lattice structures that can accommodate only minor amounts of silver bromide and/or chloride. Difficulties with development have severely limited the use of these grains for latent image capture in photography.
Silver chloride and silver bromide each forms a face centered cubic rock salt crystal lattice structure. All relative proportions of chloride and bromide ions can be accommodated in this crystal lattice structure. Iodide ion can be accommodated up to its saturation limit, which is approximately 40 mole percent, based on total silver in a silver bromide crystal lattice structure, and up to about 13 mole percent, based on silver in a silver chloride crystal lattice structure, the exact limit varying within a few percent, based on temperature.
A large proportion of photographic emulsions contains silver iodohalide grains, that is, grains in which a significant, performance modifying concentration of iodide is contained in a face centered cubic rock salt crystal lattice structure formed by one or both of the silver chloride and bromide. The highest levels of photographic sensitivity are typically realized by providing high bromide grains containing a minor amount of iodide, such as silver iodobromide grains. The presence of minor amounts of iodide ion can also enhance the sensitivity of high chloride grains. It is disclosed in U.S. Pat. Nos. 5,547,827; 5,726,005; 5,736,310; and 5,728,516 that iodochloride emulsions may be formed that have improved speed. These emulsions have the iodide incorporated at or below the surface of the grains.
To appreciate the techniques and difficulties for preparing mixed halide grains that contain iodide, it is necessary to appreciate the relative solubilities of the different photographically useful silver halides.
Although the majority of the silver and halide ions are confined to the grains, at equilibrium a small fraction of the silver and halide ions is also present in the dispersing medium, as illustrated by the following relationship: ##STR1## where X represents halide. From relationship (I) it is apparent that most of the silver and halide ions at equilibrium are in an insoluble form, while the concentration of soluble silver ions (Ag.sup.+) and halide ions (X.sup.-) is limited. However, it is important to note that equilibrium is a dynamic relationship, that is, a specific halide ion is not fixed in either the right-hand or left-hand position in relationship (I). Rather a constant interchange of halide ion between the left- and right-hand positions is occurring.
At any given temperature the activity product of Ag.sup.+ and X.sup.- is at equilibrium a constant and satisfies the relationship: EQU Ksp=[Ag.sup.+ ][X.sup.- ] (II)
where Ksp is the solubility product constant of the silver halide. To avoid working with small fractions, the following relationship is also widely employed: EQU -log Ksp=pAg+pX (III)
where
pAg represents the negative logarithm of the equilibrium silver ion activity and PA1 pX represents the negative logarithm of the equilibrium halide ion activity. From relationship (III) it is apparent that the larger the value of the -log Ksp for a given halide, the lower is its solubility. The relative solubilities of the photographic halides (Cl, Br, and I) can be appreciated by reference to Table A: PA1 R is the residue of the methionine group in the peptide chain of the gelatin.
TABLE A ______________________________________ AgCl AgBr AgI Temp. .degree. C. log Ksp log Ksp log Ksp ______________________________________ 40 9.2 11.6 15.2 50 8.9 11.2 14.6 60 8.6 10.8 14.1 80 8.1 10.1 13.2 ______________________________________
From Table A it is apparent that at 40.degree. C. the solubility of AgCl is one million times higher than that of AgI, while the solubility of AgBr ranges from about one thousand to ten thousand times that of AgI.
When silver ion and two or more halide ions are concurrently introduced into a dispersing medium, the silver ion precipitates disproportionately with the halide ion that forms the least soluble silver halide. It is therefore appreciated that the presence of local iodide ion concentration variances in the dispersing medium in the course of precipitation of silver iodohalide grains result in iodide ion non-uniformities in the grains precipitated. When the limited ability of a face centered cubic rock salt crystal lattice structure to accommodate iodide ions is taken into account, it is readily appreciated that if iodide ion non-uniformities in the dispersing medium are sufficiently large, a separate, unwanted high iodide (.beta. or .gamma. phase) grain population can be produced.
In the large scale precipitation of iodochloride emulsions, a mixing sensitivity problem arises. This occurs when KI is used as the source of iodide in precipitating the iodochloride emulsion. The rate of reaction between iodide ion and silver ion is much faster than the rate of dispersion of the potassium iodide reactant. The latter rate is dependent on the amount of the KI dispersed, the rate of blending, and the kettle volume. This results in the uneven distribution of the iodide ion from grain to grain and from batch to batch depending on the rate of mixing and thus the rate of dispersion. The resulting silver iodochloride emulsion thus varies in photographic performance and lacks manufacturability control.
As a technique for better controlling the uniformity of iodide ion availability within the dispersing medium, it has been recently suggested (see Takada et al U.S. Pat. No. 5,389,508; Suga et al U.S. Pat. No. 5,418,124; Maruyama et al U.S. Pat. No. 5,525,460; and Kikuchi et al U.S. Pat. No. 5,527,664) that the uniformity of iodide ion within the dispersing medium can be better controlled by introducing iodide in the form of a compound satisfying the formula: EQU R--I (IV)
wherein R represents a monovalent organic residue which releases iodide upon reacting with a nucleophilic reagent, such as hydroxide, or sulfite ion or ammonia. Hydroxide ion and ammonia are basic species that are known to cause a rise in pH. An increase in pH has been demonstrated to produce fog in emulsion making. Such fog formation is non-discriminatory and gives rise to poor image in the art of silver halide photography. Additionally, formation of sulfite anion, a silver halide grain ripening agent, may lead to changes in grain morphology.
U.S. Pat. No. 5,726,005 describes photographic elements containing cubical grain silver iodochloride emulsions. U.S. Pat. No. 5,736,310 teaches the preparation of cubical grain silver iodochloride emulsions and processes. U.S. Pat. No. 5,792,601 of Edwards et al discloses a process for the preparation of iodochloride emulsions with incorporated iridium dopant. U.S. Pat. No. 5,736,312 of Jagannathan et al discloses a process for introducing iodide ion into the crystal lattice of silver halide grains by reacting an iodate (IO.sub.3.sup.-) anion with a sulfite anion, a known silver halide grain ripening agent.
The organic ligand release (see formula IV above) approach for introducing iodide into silver halide grain crystal lattice structures, as well as the Jagannathan et al approach of employing iodate (IO.sub.3.sup.-) anion, has significant disadvantages. In order to release iodide ion by these methods either a strong grain ripening agent, such as sulfite ion, or an elevated pH is required. Elevated pH conditions risk undesirably elevating fog levels in the emulsions. This occurs because the conditions are favorable for a portion of the silver ions, Ag.sup.+, being reduced to Ag.degree.. When a few Ag.degree. atoms are located in close proximity, the grain can spontaneously develop, independent of its exposure. This is sometimes referred to as reduction fog or R-typing.
The requirement of a sulfite anion is particularly undesirable, since sulfite is known to act as a grain ripening agent. That is, it tends to speed the ripening out of smaller grains onto larger grains and the preferential solubilization of grain edge and corner structures. This can have an undesirable effect of changing the shape of the grains. For example, where it is desired to maximize a particular class of external crystal faces, such as {111} or {100} faces, ripening can have the effect of rounding edges and comers to decrease the proportion of clearly {111} or {100} grain faces. This same edge and corner rounding can also degrade grain shapes, such as well-defined cubic, octahedral, or tabular grains, causing regression toward spherical forms as a function of the degree of ripening that has occurred.
The use of iodate (IO.sub.3.sup.-) ion to release iodide (I.sup.-) anion, as taught by Jagannathan et al, is relatively inefficient, since three sulfite anions are required to release a single iodide (I.sup.-) anion, as illustrated by the following equation: EQU IO.sub.3.sup.- +3SO.sub.3.sup..dbd. .fwdarw.I.sup.- +3SO.sub.4.sup..dbd.(V)
Thus, to arrive at a 3 mol percent iodide concentration in the grains by the process of Jagannathan et al, it is necessary to introduce nearly 10 mol percent sulfite ion, based on silver. This is a high proportion of sulfite ion.
Finally, the water solubility of iodine is very limited. At 20.degree. C., iodine is soluble in water only at 0.029 g per 100 mL (Handbook of Chemistry and Physics). To achieve a higher solubility, the use of alcoholic solvents are suggested. However, the use of these organic solvents are environmentally hazardous and are not recommended. Additionally, iodine is a very volatile solid. It sublimes easily at room temperature to the vapor state. This volatility makes it difficult to control the exact quantities needed in the large-scale manufacturing of AgICI emulsions.